The efficient plastic packaging of products of many different shapes, chemical nature and purpose is a significant challenge. Keeping food and drink fresh is a particularly demanding sector where shelf life can vary widely from product to product and serving size can vary considerably. The greatest cause of spoilage in food and drink is oxygen infiltration leading to oxidation of the product. Most common packaging materials which are aesthetically satisfactory for thin film packaging are very poor at stopping gas transfer into and out of packaged food. Over the last thirty years an industry has developed in providing barrier film layers. These films are used to stop the transfer of gases such as water vapour, O2, CO2 and N2 into and out of food and beverages.
Plastic has been used as a packaging material for over sixty years and is continuing to evolve through increasing demands from the market and through technical development. The commodity plastics such as polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC) and polyethylene (PE) all have some barrier properties against the ingress or egress of O2 and H2O. These barrier properties are usually proportional to the thickness of the barrier layer. Three factors drive the need for plastics with better barrier properties; firstly the move away from glass and tin/aluminium because of their weight, cost and in the case of glass, its fragility; secondly the down gauging of plastic materials to make them more economic, and thirdly the requirement for extended shelf life as more food is packaged into ever smaller serving sizes.
These factors have led to the development of materials that significantly enhance the barrier properties of common consumer plastic packaging. The first successful high performance barrier material was polyvinylidene chloride (PVDC). It is a derivative of PVC and therefore is seen to have a similar negative environmental profile. The other common barrier materials on the market today are ethylene vinyl alcohol copolymer (EVOH), nylons (e.g. MXD6) and nitriles. These are all used as a barrier layer in addition to the structural layer provided by the commodity plastics.
The only natural polymer that has been commercialized that can be used as a barrier material is cellophane which was developed well before much of the plastics industry. Its barrier properties to oxygen are not considered high performance compared to today's barrier resins and its cost is high.
A common barrier structure of the wall of PET (polyethylene terephthalate) drink bottles is a multi-layer structure consisting of PET structural layers around a core layer or layers containing higher-priced barrier materials. U.S. Pat. Nos. 5,498,662, 5,621,026, 5,897,960 and 6,143,384 disclose the use of polymethacrylic acid polymer and polysaccharides in barrier layers. WO 00/49072 discloses barrier coatings based on clays such as montmorillonite spray coated onto PET blow moulded bottles. USA application 2004/0087696 discloses a water based coating for PET containers in which a clay material is mixed with a melamine, formaldehyde and boric acid binder and an organic water soluble binder such as polysaccharides and cellulose materials.
Barrier materials are used in a host of different plastic structures and processes and each of these imposes its own functionality requirements. The most common use of barrier structures is in thin films for wrapping foods such as confectionery, fresh food, bakery products and the plethora of pouches of instant, convenience foods such as flavourings and dehydrated sauce bases which have appeared in the market in recent years. Some of these films are highly complex laminates which can have 12 layers and yet be less than 50 μm thick. Lamination of plastics is also challenging. These films are typically produced through co extrusion. WO 90/14938 discloses a high amylose modified starch that is suitable for use in oxygen barrier laminates. U.S. Pat. Nos. 6,569,539 and 6,692,801 disclose a paper and/or plastic laminate with an inner barrier coating of a starch or modified starch applied from a dispersion. WO04/052646 discloses a multilayer barrier film using a starch layer and a biodegradable polyester layer. USA application 2002/0187340 discloses a gas barrier coating of polyvinyl alcohol and starch in which the predominant material is starch and the material is applied from a dispersion.
Barriers are also used in bottles for fruit juice, in some carbonated soft drinks and in various hot filled foods such as fruit and vegetable preserves. Bottles are usually formed through co-injection stretch blow moulding. This requires the materials to be both injection moulded into a preform and then remelted and blown into a bottle shape. Other containers may be co-extrusion blow moulded, where the parison is blown against a mould wall to achieve the desired shape online during the co-extrusion process.
Some containers further require high gas-barrier closures, formed by means of injection moulding.
Barrier materials are also used in rigid packages such as meat trays, although for most applications the rigid plastic material provides enough barrier and only the thin film on top needs improved performance.
Barrier films may also be used for such diverse applications as silage wrapping and the packaging of rolled newspapers for home delivery.
One issue that has slowed the rollout of barrier technologies is their effect on the recyclability of the plastic—this is particularly true for the bottle market. Many PET bottles now have a complex structure of virgin material on the exterior with recycled PET and barrier layers in the middle. If the barrier resin is not compatible with the recycling system then there can be considerable resistance to adoption of that technology. New materials are entering the market that are based on sustainable, renewable resources and/or that are biodegradable. An example of such a material that may be injection stretch blow moulded into bottles or formed into thin films for packaging applications is polylactic acid (PLA) synthesised from corn. PLA has poor gas barrier properties and also relatively poor water vapour barrier properties, and in order to preserve biodegradability or sustainability status would benefit from being used with a biodegradable barrier based on renewable resources.
Another issue in manufacturing barrier films is the selection and management of appropriate plasticizers. A plasticizer is a substance added to polymeric materials to promote flexibility, workability, and elongation. For barrier films to work well, they must be very dry. Temperatures commonly used to satisfactorily dry the films can be in the order of 90-280° C. and higher. At these temperatures, the plasticizers either evaporate out or are distilled out as the water in the composition escapes. Low molecular weight plasticizers are particularly problematic as they have a measurable vapour pressure at drying and processing temperature. The addition of a humectant does not always assist in managing the plasticizers as at higher processing temperatures the capacity of the humectant to attract and retain moisture is not sustained. A further issue can arise during and even after processing. Plasticizer migration occurs when the plasticizer moves from the body of the plastic into the surface of the film or into other components of the plastic composite or laminate such as an adhesive, face material, or both. This can cause adhesion, processing and cosmetic problems such as bubbles and holes in the film and bleed-through of adhesive components into the face material.